Rep. Heather Wilson, R-New Mexico, appears with President Bush at an Albuquerque fundraiser in 2006.

Wilson's Albuquerque district was a top take-over target for House Democrats during the 2006 elections. "She's got a terrible district now," noted one GOP source. "She'd do much better in a statewide race."

Wilson, a senior member of the House Intelligence Committee, was elected to a sixth House term in 2006.

The GOP sources also tell CNN that Republican Rep. Steve Pearce, who is considered more socially conservative than Wilson, may also decide to run for Domenici's seat.

A source close to Pearce said he "is strongly considering a run for the Senate."

Domenici, a Republican, announced Thursday that he won't seek re-election next year because he suffers from a progressive, incurable brain condition that might prevent him from completing another six-year term.

"I had to consider whether I could, in good conscience, run for re-election and serve you as well as you deserve," said Domenici, 75, who made the announcement in the gym of an Albuquerque grammar school he attended as a boy.

"I concluded it would be wrong to ask New Mexicans to support me if I could not pledge that I could ably serve another full term."

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Domenici, first elected to the Senate in 1972 and re-elected five times, said he has been suffering for the last two years from frontotemporal lobar degeneration, or FTLD, a deterioration of brain tissue that can lead to personality changes, difficulty with speech and dementia.

While the condition has had "very little impact" on him, Domenici said a checkup in September showed a "slight" progression of the condition. He said the "erratic, unpredictable" nature of the illness prompted his decision to retire.

However, the senator said he is still well enough to do his job and has "no doubts" he can finish out the rest of his current term, which ends in January 2009.

"We're going to do our very best with those 15 months to do something spectacular," he said. Domenici concluded his remarks with a call for more research into brain diseases. "No cure for my disease exists yet, but if we work hard enough, we may be able to find a way to cure people with diseases of the brain," he said. "That would be a wonderful thing in the future."

Wilson, like Domenici, has faced criticism for her role in the controversial firings of U.S. attorneys.

One of the prosecutors, David Iglesias, contends Domenici and Wilson wanted him to push harder on a corruption investigation of New Mexico Democrats before the 2006 midterm elections.

Iglesias said he felt "leaned on" when Domenici called him to inquire about when indictments would be brought. The senator, who nominated Iglesias for the post, apologized for making the call but said he never pressured Iglesias.

Domenici joins three other Republican senators who also are retiring at the end of their terms next year, and, with four other Republican senators facing difficult re-election bids, the numbers are stacked against GOP hopes of recapturing the upper chamber of Congress.

Democrats took back both houses of Congress last November, but hold thin majorities. In the Senate, they have a 51-49 margin.

Their advantage in the House of Representatives is 233-202. Of 34 Senate seats being contested next year, Republicans hold 22

Republicans also face tough competition to hold onto the seats of retiring senators in Nebraska, Virginia and Colorado.E-mail to a friend